1. Field of the Invention
The present invention relates to a multimedia television receiver to which signals having different deflection frequencies and/or in particular different flyback periods including the ordinary NTSC television signal and the video signal from personal computers are input and which displays such input signals on a common CRT display.
2. Description of the Related Art
A multimedia-supporting color television set which can display not only the video images from various video media including television, video tapes, CATV and laser disks but also images from various personal computers (such as multimedia personal computers with incorporated CD-ROM drives) on a single video receiver have been anticipated. In such a color television set, the video signal from general television broadcasting and a video tape player (NTSC signal) or those from a personal computer (RGB signal) is selectively inputted.
A video signal from a personal computer, however has more information than that from television broadcasting or a video tape. The video signal also has a wider frequency band and a different flyback period. For example, while the NTSC signal has a vertical flyback period of 1080 .mu.s, the video signal from a personal computer has a quite short flyback period: 600 .mu.s. Thus, there is a large difference. For reproduction of the video signal from a personal computer on a CRT display in particular, a pump-up control may be required to raise the wave height of the deflection voltage during the flyback period in order to smooth flyback operation by the deflection yoke of the CRT display.
Specifically, the display is designed so that the deflection yoke current during the flyback period returns from "-1/2i yp-p" to "+1/2i yp-p" (where "i yp-p" is the peak-to-peak value of the deflection yoke current) in a short time and a pump-up voltage higher than that for the scanning period is supplied to the deflection yoke during the flyback period. Note that the loss at the output stage increases if the deflection yoke voltage is pumped up for a higher voltage even during the scanning period. Therefore, it is necessary to make sure that the pump-up voltage is applied to the flyback period only.
FIG. 7 shows an example of a conventional vertical deflection circuit provided with a pump-up circuit. The power supply voltage Vcc is supplied to the transistors Q1 and Q2 constituting a vertical output circuit connected in series via the diode D1 for reverse current prevention. These transistors Q1 and Q2 are controlled by the vertical drive signal and the vertical deflection signal is supplied to the deflection yoke DY. The output from the diode D1 charges the capacitor C1, which is connected to the reference potential point via the change-over switch SW during the scanning period of the input signal. This change-over switch SW is designed so that the capacitor C1 is connected to the power supply Vcc during the flyback period.
In such a pump-up circuit, the change-over switch SW connects, as shown in FIG. 7, the capacitor C1 to the reference potential point during the scanning period so that the capacitor C1 is charged to the voltage almost equivalent to Vcc expressed as "Vcc-VF" (where "VF" is the forward voltage drop of the diode D1) and the power supply voltage Vcc is applied to the deflection yoke DY. On the other hand, the change-over switch SW is switched so that the discharged voltage from the capacitor C1 is additionally supplied to the output stage during the flyback period. Thus, the voltage "Vcc-VF" is added to the power supply voltage Vcc and the total voltage, which is about 2 Vcc, is applied to the output stage so that the deflection yoke current is restored to the current "+1/2i yp-p" in a short period.
Since the video signals from personal computers have vertical deflection frequencies and the flyback periods which differ from the NTSC signal, the pump-up voltage during the flyback period upon receiving an input of the video signal from a personal computer is required to be higher than "2.times.Vcc", which is for input of the NTSC signal. Conventional solutions to this problem are as follows:
a) The pump-up voltage is always kept to three times the voltage Vcc (Vcc.times.3); PA1 b) The pump-up voltage is "Vcc.times.2" as in the case of the NTSC signal, but the deflection yoke is provided with a smaller inductance; PA1 c) A deflection yoke with a poor deflection sensitivity leads to a longer flyback period, which causes the vertical deflection current to go into the scanning period of the signal, resulting in folding on the screen. In such case, blanking is executed utilizing the pulse from the output stage of the vertical output circuit.
However, the solution a) results in a larger loss and b) results in a large deflection yoke current due to small inductance of the deflection yoke, which results in a larger loss at the output stage and a higher temperature at the deflection yoke. In addition, difference of the flyback period between the NTSC signal and the personal computer signal causes the vertical screen position to be shifted upward. Therefore, it is necessary to correct it by adding a centering circuit as shown in FIG. 8. In FIG. 8, the DC voltage (Vcc) is divided by the resistance circuit (R2 to R4) and the DC component is supplied via transistors Q3 and Q4 to the vertical deflection yoke DY for adjustment of the screen position. In case of c), since the scanning period or the period for images is reduced, the information amount is naturally restricted.
FIG. 9 (A) shows the vertical synchronizing signal in the video signal. In the vertical flyback period L1 of the video signal, the vertical flyback period L2 of the vertical deflection circuit is arranged. This vertical flyback period L2 is 1080 .mu.s for the NTSC signal and 600 .mu.s for the personal computer signal. The vertical deflection signal supplied to the deflection coil is, as shown in FIG. 9 (B), configured corresponding to this vertical flyback period L2. With a deflection yoke having a poor deflection sensitivity, for example, the flyback period becomes longer and goes into the image section of the video signal as shown in a.
Thus, a conventional multimedia television receiver has drawbacks that, the pump-up solutions for input of a signal with a different vertical deflection frequency or the vertical flyback period width such as the personal computer signal results in a large loss at the output stage, requires a screen position adjustment circuit or reduces the scanning period.